The invention concerns a circuit for adjusting the impedance of a plasma section to a high-frequency generator.
When producing plasma with a high-frequency generator, e.g., a generator oscillating at 13.56 MHz, the problem arises that a portion of the high-frequency energy does not go into the plasma, but is reflected back to the generator since the generator cannot be continuously adjusted to the plasma section. Adjustment difficulties are caused, for example, by the fact that the plasma impedance changes perpetually by approximately a factor of 2, while the matching impedance of the generator is constant. For instance the impedance of a 13.56 MHz generator normally comes to 50 Ohms. Much greater matching problems arise when the electrodes are reversed and impedance changes of several powers of ten occur, i.e., between 10 Ohms and 1000 Ohms. Optimal wattage match thus results only at a few impedances of the plasma; at all other impedances, reflective phenomena appear. One way to optimize the wattage match could consist of holding the plasma impedance constant. However, this would involve extremely high expenses, since the plasma impedance is dependent on numerous parameters such as the geometry of the unit, the type of electrode, the screening, gas feeds, type of gas and gas pressure.
A device for the production of HF-induced inert gas plasma is already known, in which a steadily burning plasma can be produced (German Patent No. DE-OS 36 38 880). The energy required here to ignite and maintain the plasma is excited capacitatively via two opposing capacitor plates between which the plasma is formed and/or located. The capacitor plates which form an oscillatory circuit together with an inductor, are supplied with HF voltage, the frequency of which is the same as the resonant frequency of the oscillatory circuit. Wattage matching with variable plasma impedance is not possible with this known device.
In another known device for HF-induced dry etching, the ratio between the total HF wattage of an HF generator and the power actually converted into wattage is determined (van der Hoek, de Vries and Heljman; "Power Loss Mechanisms in Radio Frequency Dry Etching Systems," J. Vac. Sci. Technol. B 5(3), May/June 1987, pp. 647-651). In this device, the HF wattage is fed into the plasma reactor through an accommodating connection, since the initial impedance of the generator is normally 50 Ohms, while the plasma impedance is generally capacitative at a high frequency. The matching network is thus required to maintain an optional power transfer. In any event, an L-circuit is provided in the known device for wattage matching, which is suitable only for a limited range of the plasma impedance.
Furthermore, a procedure for regulating a plasma etching process is known, in which the impedance changes in HF wattage are indicated (U.S. Pat. No. 4,207,137). Here a standard LC matching circuit is used, corresponding to the above-mentioned L-circuit.
A sputtering device is also known, in which an HF voltage is superimposed on direct current; it has a matching circuit (F. Vratny, "Deposition of Tantalum and Tantalum Oxide by Superimposed RF and DC Sputtering," J. Electrochem. Soc.: Solid State Science, Vol. 114, No. 5, 1967, pp 505-508). The matching circuit for the sputtering device here is created through a connection and/or a transformer coupling in the form of an L-circuit, and T- and Pi-members are also indicated as possible, with reference to the state of the art (B. Goodman, The Radio Amateur's Handbook, American Radio Relay League, Newington, Conn., 1966).
Further, a Cauer LC high-pass is also known, in which three capacitors are provided on the series arm and in which series circuits of one capacitor and one inductor are provided in the cross arm located between the capacitors (W. Rienecker, Elektrische Filtertechnik [Electrical Filtering Techniques], 1981, p. 100).
Finally, a circuit for the wattage matching of a high-frequency generator was proposed in which a T-member is provided between the high-frequency generator and the plasma section, which has two variable series capacitors and one fixed shunt inductor (German Patent Application P 38 21 134.2). This known circuit is less suitable for high load impedances, since it then requires a coil with high inductance (&gt;2000 nH), which is relatively bulky.
The invention is therefore based on the task of creating a matching circuit suitable for high load impedances.
This task is solved by the characteristics set forth in claim 1. The advantage attained with the invention consists particularly in the fact that even at high load impedances, relatively low inductances (approximately 500 nH and 750 nH) suffice, whereby the whole matching circuit can be built much more compactly. An embodiment of the invention is presented in the accompanying drawings and described in greater detail below.